In “electronic watermark” with information for preventing copying and counterfeiting and confidential information invisibly embedded, storing and data passing are assumed to be carried out on electronic media, which makes it possible to detect information reliably without deterioration and loss of information embedded watermarked. Similarly to this, there is required a method of embedding confidential information, which is not visually unsightly in a form of other than character and cannot be falsified easily, in a printed document in order to prevent alteration and copying of a document printed on a paper medium.
As an information embedding method for monochrome binary document used most widely for a printed matter, the following technology is known.
[Patent Document 1] JP-A-2001-78006 “METHOD AND DEVICE FOR EMBEDDING AND DETECTING WATERMARK INFORMATION IN BLACK-AND-WHITE BINARY DOCUMENT PICTURE”
A minimum rectangle surrounding an arbitrary character string is divided into some blocks, which are divided into two groups (group 1 and 2) (the number of groups may be three or more). When a signal is 1, for example, a feature quantity in each block of group 1 is increased while a feature quantity in a block of group 2 is reduced. When a signal is 0, a reverse operation is carried out. The feature quantity in a block is such as the number of pixels and weight of character in a character area, a distance from the point of scanning the block vertically and to the point of reaching the character area first.
[Patent Document 2] JP-A-2001-53954 “DEVICE AND METHOD FOR EMBEDDING INFORMATION AND READING INFORMATION, DIGITAL WATERMARK SYSTEM AND RECORDING MEDIUM”
Setting the width and height of the maximum rectangle surrounding one character as the feature quantity for the character, a symbol is assumed to be indicated by a classified pattern of magnitude relation of the feature quantity among two or more characters. For example, six feature quantities can be defined from three characters, and listing the combinations of the patterns of magnitude relation, and classifying these combinations into two groups to give a symbol. When the information to be embedded is “0” and the combination pattern of the feature quantity of character selected to indicate this is “1”, one of the six feature quantities is varied by expanding its character area. The pattern to be varied is selected in order for the amount of varying to be minimum.
[Patent Document 3] JP-A-09-179494 “CONFIDENTIAL INFORMATION RECORDING METHOD”
It is assumed that a printer with 400 dpi or more is used for printing. Information is quantified to express the information by a distance from a reference point mark to a position determination mark (dot number).
[Patent Document 4] JP-A-10-200743 “DOCUMENT PROCESSING UNIT”
Information is expressed according to whether a screen line of a multi-line screen (a special screen configured by minute parallel lines) is moved backward or not.
In the Patent Documents 1 and 2, however, changing font and layout becomes required due to the changes of pixel configuring the character of document image, character spacing and line spacing. In the Patent Documents 3 and 4, in addition, since there is required high-accuracy detection process per pixel of the input image read out from an input device such as scanner in detecting, dirt on paper and addition of noise in printing and reading have a great impact on information detecting accuracy.
In the Patent Documents 1 to 4, as described above, when detecting confidential information embedded by inputting the printed document in a computer again by an input device such as scanner, image deformation caused by dirt on the printed document and rotation generated in inputting allow a noise component to be included in the input image, which makes it difficult to extract the confidential information correctly.
As the case of having a large impact on the information detecting accuracy with regard to the Patent Documents 3 and 4, there are the case of inclining of the printed document in reading from an input device and the case of local expansion and contraction of image due to displacement of paper in printing or inputting an image.